Low-carbon energy production is potentially a major method of reducing greenhouse gas emissions and anthropogenic climate change. In the UK, tall perennial grass crops show potential as “biomass crops”, providing renewable energy sources with a low net carbon cost. However, conversion of large areas of farmland to biomass production would constitute a major land-use change with possible negative effects on native biodiversity, particularly as some biomass crop types are not native to the UK. The aim of this thesis was to assess biological diversity within mature (>3 years old) crops of non-native Miscanthus x giganteus and native Phalaris arundinacea.\ud \ud Biomass crop structural characteristics and management regimes were recorded, and their biodiversity was surveyed with particular reference to birds and small mammals in comparison with adjacent land uses. Food resources in terms of non-crop vegetation and invertebrates were also recorded. \ud \ud Live-trapping revealed eight species of small mammal in the study crops, including a conservation priority species, the harvest mouse Micromys minutus, which was most abundant in Phalaris crops. Phalaris also contained the highest small mammal diversity, but the field headlands held the greatest small mammal abundance. Trapping and direct observations revealed a higher abundance and diversity of birds in the Miscanthus crops in comparison with Phalaris. Most of the bird species found in biomass crops were associated with woodland or reedbed rather than farmland habitat.\ud Phalaris crops had a higher percentage of ground cover of the crop itself and non-crop vegetation, whereas Miscanthus fields had greater cover of crop litter. Miscanthus crops contained fewer invertebrates than Phalaris or the field headlands. Management specific to biomass grass crops involves harvest in spring, thus providing winter habitat of importance to birds and small mammals. The crop fields also provide a refuge for invertebrates and non-crop vegetation and overall, supported high levels of biodiversity.